Photonics Solves the Wafer Temperature Problem

Charles T. Troy

SANTA CLARA, Calif. -- As a semiconductor wafer wends its way through the fabrication process it passes through several stages where thermal processing plays a key role.
Conventional techniques require that manufacturers stop the process, remove the wafer from the fabrication process and make a measurement off line.
Now a photonic technique developed by Luxtron and Lucent Technologies promises to bring an end to start-and-stop thermal measurements. In doing so, it should significantly reduce the productivity losses that result from the awkward process.

'Ripple' technology
The in situ, real-time process employs so-called "ripple" technology in which a light source, often incandescent or quartz-halogen, teams with fiber optics and detectors to interrogate the wafer.
The key to the technology is plain old 60-Hz AC wall-plug power. Because 60-Hz AC oscillates across a zero mark, intensity can be measured. And because the temperature of a substrate affects that intensity, it can be correlated to temperature.
Chuck Schietinger, Luxtron's manager of strategic development, told Photonics Spectra that the system employs two sapphire fibers. One monitors the "ripple," and the other acts as a reference. In addition, two detectors and two amplifiers complete the system. Detectors can be silicon, indium gallium arsenide or indium arsenide, depending on which temperature range is of interest.
In operation, one sapphire fiber looks at the wafer, while the other monitors the background light. The modulation of the background light is compared with that being "rippled" by the heated wafer.

Processing data
The data gleaned by the detectors are then processed by software developed jointly by Lucent and Luxtron. The system can measure wafer temperatures from approximately 350 °C to 1200 °C.
The project began when Lucent obtained a number of systems from Luxtron for use on rapid thermal processing furnaces. Consequently, both companies worked on improving the design, with Lucent building additional systems under license from Luxtron. The two firms recently signed a cross-licensing agreement under which Luxtron will manufacture, sell and support the product.